The invention of this application relates to ink jet printing compositions and their use as color change indicators. The compositions are particularly useful as sterilization and/or thermal exposure indicators.
Ink jet printing is a well known technique by which printing is effected without contact between the printing device and the substrate on which the printed characters are deposited. Briefly described, ink jet printing involves the technique of projecting a stream of droplets of ink to a surface and controlling the direction of the stream electronically so that the droplets are caused to form the desired printed image on the substrate. That technique of non-contact printing is particularly well suited for application of characters onto irregularly shaped surfaces, including, for example, the bottom of beverage containers.
In general, ink jet printing techniques impose rigid requirements on the ink compositions. To be suitable for use as a jet ink, the compositions must meet rigid requirements of viscosity and resistivity, solubility, compatibility of components and wettability of substrate; the ink must be quick-drying and smear resistant, must be capable of passing through the ink jet nozzle without clogging and must permit rapid clean-up of the machine components with minimum effort.
Apart from the jet ink applications of the ink, another area of interest in the art is the provision of printable pasteurization or sterilization indicators. Such compositions have heretofore usually utilized pigments and pigment combinations and have been applied primarily in determining whether proper sterilization heat has been applied to objects used in medical and surgical procedures. In the food packaging industry, for example, where metal cans, plastic pouches or coated metal cans are printed, filled with product, sealed and the package subjected to conditions of high temperature and moisture during pasteurization or sterilization techniques, there is a need for such compositions that are printable by either contact or jet ink techniques, that exhibit sufficient adhesion to the substrate to withstand the moisture and high temperature conditions of sterilization and which, at the same time, undergo a visible and permanent color change.
The provision of such an ink composition which shows a distinct difference in color between an unsterilized and a completely sterilized package provides for ready visible inspection and permits tracing a particular package or packages after they have been processed. The invention thus provides a positive and visible indication that sterilization or pasteurization has in fact been carried out on the product to which the ink is applied.
Thermochromic inks and paints have been known for some time.
U.S. Pat. No. 3,471,422 describes a temperature responsive printing ink which is responsive to autoclaving. This ink is based on a pigment which initially is of one given color and which upon exposure to the elevated temperature condition reacts with a sulfur generated by a second component of the ink to generate a second color.
U.S. Pat. No. 4,166,044 describes a temperature responsive jet printing ink useful as a sterilization or pasteurization indicator. This ink comprises either (1) a colorant which reacts under the elevated temperature conditions to change its color, or (2) a combination of colorants which together form an initial given color and under steam sterilization conditions changes color due to leeching.
Similarly, U.S. Pat. No. 4,179,397 discloses an extractable/nonextractable dye combination for thermotropic ink compositions which meet the rigid requirements for ink jet printing techniques. Finally, U.S. Pat. No. 4,155,895 performs similarly to the thermotropic ink described in U.S. Pat. No. 4,179,397 but that rather than relying on the leeching out of a more soluble dye at elevated temperature in the presence of steam or water the '895 patent utilizes a thermochromic dye that reacts at elevated temperature in the presence of steam to undergo a visible color change.
These prior art inks show a color change upon exposure to a temperature level/time period either because (1) there is a change in the molecular structure of the color component and the new molecule displays a different color, or (2) a color component is selectively extracted to enable the ink to show as a new color.
It is, therefore, a principal object of the present invention to develop an ink which indicates by a color change that it was exposed to an environment having a temperature in excess of a predetermined level for at least a specified minimal period of time. It is an additional object of the present invention to exploit the inherently more temperature specific process of chemical degradation in this case, of heat-sensitive dyes, to show more precisely and dramatically the exposure of an article printed with an indicator ink to a predetermined temperature level/time period. Another object of the present invention is to make a jet printable ink that could display a desired message having a predetermined color on a container or package in response to a specific temperature level/time period.
These and other objects of the invention will be apparent from the description which follows.